Photochemical conversion of CO to C1 and C2 products mediated by porphyrin rhodium(II) metallo-radical complexes.

Unimolecular reduction and bimolecular reductive coupling of carbon monoxide (CO) represent important ways to synthesize organic feedstocks. Reductive activation of CO through open-shell pathways, though rare, can help overcome the barriers of many traditional organometallic elementary reactions that are hard to achieve. Herein we successfully achieve the unimolecular reduction of CO to (TPP)RhCHOSiRRR (TPP = 5,10,15,20-tetraphenylporphyrin), and the release of products CHOSiRRR, TEMPO-CHOSiRRR and BrCHOSiRRR in near-quantitative yield under visible light (420-780 nm), which involves radical formation from Rh-C bond homolysis. Bimolecular CO reductive coupling products, (TPP)RhCOCHOSiRRR, are then obtained via a radical mechanism. Subsequent treatment with n-propylamine, BrCCl or TEMPO under thermal or photochemical conditions afford small-molecule bimolecular reductive coupling products. To the best of our knowledge, homogeneous systems which reductively couple CO under photochemical conditions have not been reported before. Here, the use of an open-shell transition metal complex, that delivers more than one kind of small-molecule CO reductive coupling products bearing different functional groups, provides opportunities for useful CO reductive transformations.